1. Field of the Invention
The present invention relates to a sound field measurement device for determining the number of people and their positions in a sound field where an audio signal is outputted and for measuring the reverberation time of the sound field.
2. Description of the Background Art
When an audio signal is reproduced from a CD or a DVD in a room (e.g., a listening room, or an automobile cabin), there are usually one or more listeners in the room, i.e., in the sound field. Since the listeners are inevitably present at different positions (they cannot physically be present at exactly the same position), it would be desirable if the tone quality, the sense of sound field, the sense of sound localization, etc., can be adjusted optimally for the number and positions of the listeners. Since a human is by nature a sound absorber, the reverberation time of a sound field varies depending on the number of people present therein. The reverberation time also varies depending on the interior finish of the room. Therefore, the reverberation time should also be adjusted optimally. To do so, it is necessary to determine the number and positions of people in the sound field, and the reverberation time.
It is of course possible by using a special measurement device, but such a device is expensive, and it requires a complicated process and a high level of expertise to be able to use such a device. At present, such a device has not been in general use as a consumer product. Measurement of an in-cabin sound field performed in connection with the use of a car audio system has also been a service rendered by a professional at a specialty shop. In such a service, the measurement is done at a single position using a single microphone. Measurement at a plurality of positions needs to be done while moving the microphone from one position to another. Thus, if fixed microphones are to be used, one microphone is needed for each listener (or each seat).
In a conventional approach, the audio signal adjustment is done by detecting the passenger position using a passenger sensor or a seat position detector capable of physically detecting the position of an object, instead of using a microphone for detecting an acoustic signal (see, for example, Japanese Laid-Open Patent Publication Nos. 2002-112400 and 7-222277).
In another conventional approach, passenger detection is done by using a microphone installed in a sound field. It is important in this conventional approach that the microphone is installed at a position such that sound outputted from a speaker toward the microphone is blocked by a passenger when seated, whereby the presence/absence of passengers is determined based on the level of the detection signal obtained by the microphone. Thus, the passenger detection is based primarily on the change in the direct sound portion of the sound outputted from the speaker (see, for example, Japanese Laid-Open Patent Publication No. 2000-198412).
With the seat position detection, however, the presence/absence of a passenger cannot be detected. With the passenger sensor, which does not detect the change in the sound field itself, it is not possible to know how sound-absorbing a passenger is, how much the tone quality is changed, or how much the sound field is influenced by a piece of sound-absorbing luggage present in the automobile.
Moreover, one microphone is needed for each passenger, and only one microphone is used for the detection of each passenger. Therefore, if the microphone is installed at a position where it is strongly influenced by the sound field, there will be an increased error in the level of the signal detected by the microphone. Moreover, the determination is based only on the signal level, and no description is found as to the level fluctuation due to a change in the volume level of the sound outputted from the speaker. Furthermore, since the detection is based primarily on the direct sound, changes in the reverberation characteristics cannot be known.